1. Field of the Invention
The present invention relates to a technique of adaptive equalizing radio signals such as a frequency modulated (FM) signal, and more specifically to an adaptive equalizer with a function of stopping an adaptive equalization processing.
2. Description of Related Art
In a radio broadcasting system such as an FM radio or a wireless communication system, a transmission signal is deteriorated due to multipath distortion or noises in a wireless transmission path. To that end, an equalizer for decoding the received transmission signal that is deteriorated after propagating through the wireless transmission path is used on the wireless receiver side. In general, a state of the wireless transmission path is uncertain on the receiver side. Further, a wireless receiver of high mobility should follow change in characteristics of the wireless transmission path. Thus, an adaptive equalizer capable of following a change in characteristics of the wireless transmission path has been used.
In general, the adaptive equalizer is configured by a digital filter such as an IIR (Infinite Impulse Response) filter or an FIR (Finite Impulse Response) filter, and an adaptation algorithm such as an LMS (Least Mean Square) algorithm or an NLMS (Normalized LMS) algorithm has been employed to optimize filter coefficients.
To optimize the filter coefficients based on the adaptation algorithm, a reference signal for estimating characteristics of the wireless transmission path is necessary. As one conceivable method, a training signal with a known signal pattern is sent earlier than an information signal and used as a reference signal to determine filter characteristics. As another method, a blind equalization method has been well known. The blind equalization method generates a reference signal from a received signal and thus does not need to previously send a training signal. Examples of the blind equalization method include a CMA (Constant Modulus Algorithm).
The CMA is an algorithm in general, which sets a statistical quantity regarding the filter output signal such as an envelope of a filter output signal and higher-order statistical quantity thereof as an index, and updates the filter coefficients to approximate the index to a target value. The CMA is effective for a modulation system where an amplitude of a modulation signal such as an FM (Frequency modulation) signal or PM (Phase modulation) signal is constant, in other words, a modulation signal has constant amplitude characteristics.
Japanese Unexamined Patent Application Publication No. 2005-167717 discloses an improved one of the CMA-based blind adaptive equalizer. To be specific, a multipath distortion eliminating filter is disclosed, which aims at eliminating multipath distortion of an FM signal or PM signal, and controls filter coefficients of a digital filter such that an error between an envelope of an input signal and an output of a digital filter (hereinafter referred to as “correction error”) approximates to 0.
Further, Japanese Unexamined Patent Application Publication No. 2005-167717 describes that if it is difficult to converge the correction error to 0 in the case where correction error exceeds a predetermined threshold value, for example, in such a case that a delay of a multipath signal increases beyond the total delay time of delay elements in the multipath distortion eliminating filter, fluctuations of the filter coefficients are constrained by restricting the maximum value of the correction error. The filter operation can be stabilized based on the LMS algorithm if it is difficult to make a correction error converge to 0.
Further, as another configuration example of the multipath distortion eliminating filter, Japanese Unexamined Patent Application Publication No. 2005-167717 describes the configuration for stabilizing filter operations by approximating all tap factors of a digital filter to 0 if a correction error exceeds a predetermined threshold value (paragraphs [0065] to [0067]).
As described above, the adaptive equalizer (multipath distortion eliminating filter) as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 determines a difficult situation in which the correction error between an envelope of an input signal and a digital filter output converges to 0 based on whether or not the correction error exceeds a predetermined threshold value. Further, the adaptive equalizer has a feature that, if the correction error exceeds a threshold value, an operational mode of the digital filter is changed to facilitate convergence of the correction error to 0. That is, additional processing is performed to facilitate convergence of the correction error to 0.
However, the adaptive equalizer as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 continuously performs adaptive control of filter coefficients in response to an input signal before and after an operational mode of a digital filter is changed. This causes a problem that operational stability of the digital filter is not secured.
For example, input signal intensity is changed at a small interval, it is impossible to make the correction error to converge even through the CMA to approximate the correction error to 0, leading to unstable operations such as oscillations in filter coefficients.
In short, the adaptive equalization based on the CMA can adaptively configure a filter having characteristics opposite to characteristics of a wireless transmission path unless a factor of signal deterioration such as multipath distortion in a wireless transmission path, an interfering wave, and noise hinders estimation of a modulation signal of a constant amplitude from a received signal. However, if there is a factor of signal deterioration that hinders estimation of a modulation signal of a constant amplitude from a received signal, adaptive equalization processing is no longer performed stably through the CMA.
Japanese Unexamined Patent Application Publication No. 2005-167717 includes no description about configuration and operation to deal with an unstable behavior such as non-convergence and oscillations of filter coefficients of a digital filter if a factor of signal deterioration that hinders estimation of a modulation signal of a constant amplitude from a received signal.